In power transmission systems, there is a need for fast circuit breakers.
Ultra-fast actuators are a new emerging technology that have been recently used as drives when there is a need of high speed actuation. One well known topology of an ultra-fast drive is the Thomson coil. A Thomson coil comprises a primary coil that induces a magnetic field, which in turn induces eddy currents in an armature. The Thomson coil has the intrinsic property of generating large impulsive forces that actuate and promptly separate the current carrying contacts of a high voltage alternating current (HVAC) circuit breaker.
A circuit breaker of this type may, together with some extra circuitry, be used as DC circuit breaker in power transmission systems such as HVDC systems, where a system may be a multi-terminal system comprising a number of converter stations. A circuit breaker operating in a multi-terminal HVDC system or HVDC grid must be able to interrupt fault currents within some milliseconds, typically, less than 5 ms. For a Thomson coil currents in the order of several kilo Amperes are therefore required to generate a magnetic flux density in the order of several Teslas. The product of the induced current densities in the armature together with the radial component of the magnetic flux density produces the required impulsive electromagnetic forces. Due to the high currents and magnetic fields involved, a Thomson coil is often energized through the use of a capacitor bank.
The main problem of these actuators is their poor efficiency. Compared to rotating electric machines that can attain efficiencies up to 99%, traditional Thomson based ultra-fast actuators have an efficiency of 5% at best. A considerable amount of the electric energy stored in the capacitor bank is unfortunately transformed into heat.
It would in view of this be of interest to raise the efficiency of an actuator that is based on a Thomson coil.